This invention relates generally to battery charging systems for automobiles, and more particularly to an apparatus and method for controlling an engine idle speed so that the charging system efffectively operates.
In battery charging systems for automobiles, a generator, such as an alternator, driven by an internal combustion engine is controlled so that a generating voltage is kept constant irrespective of the rotational speed of the engine. To this end a current fed to a field winding of an alternator is usually controlled by way of a control unit in accordance with the voltage of the battery. While the control unit causes the alternator to increase a generated output so as to maintain battery voltage constant, even when large load electrical equipment is used, such increase in the generated output during an idle state of the engine may result in an excessive load to the engine and sometimes leads to engine stall.
Furthermore, even if engine stall does not result, since the electrical generating capability of motor vehicle alternators is relatively small during the engine idle state, excessive power consumption from the battery may cause the battery voltage to drop to an undesirable low value, and therefore, battery failure is apt to occur if such abnormal condition lasts for a long period of time.
In order to prevent such problems the rotational speed of the engine at idling may be set to a relatively higher value. However, the higher the idle engine speed, the higher the fuel cosumption. In conventional battery charging systems, therefore, an idle speed control unit is employed for increasing an idle engine speed to a predetermined engine speed when it is detected that large-load equipment is in use, so that the engine speed is raised only when such equipment is being used. Although such a conventional idle speed control unit is advantageous in view of the prevention of engine stall and battery failure, since the idle engine speed control unit is responsive to only particular equipment, idle speed increase is carried out irrespective of the operating condition of other electrical equipment. As a result, fuel is wastefully consumed by the engine when total electric power consumed by other various equipment is relatively small.